Microwave isolator



Nov. 21, 1961 H. sElDEl.

MICROWAVE lsoLAToR Filed NOV. 17, 1958 A7' TORNEI iinired states @arentdldd Patented Nov. 2l, .196i

3,illi,86 MHCRWAVE ESOLATGR Harold Seidel, llanwood, Ni., assignor toBell Telephone Laboratories, incorporated, New York, NX., a corporationof New Yaris Y' Filed Nov. i7, lSS, Ser. No. 774,547 8 Claims. (Cl.3332-24) This invention relates to electromagnetic wave transmissionsystems and more particularly to transmission structures havingnonreciprocal attenuating properties for use in such systems.

The use of materials having gyromagnetic properties to obtain bothreciprocal and nonreciprocal effects in microwave transmission circuitsis Widely known and has found numerous and varied applications inpropagation structures of both the waveguide and the transmission linetypes. A rsum of early work done in this field is contained in anarticle entitled The Behavior and Application of Ferrites in theMicrowave Region by A. G. Fox, S. E. Miller and M. T. Weiss, Bell SystemTechnical Journal, January 1955, pages -103. The Proceedings of theInstitute of Radio Engineers, Volume 44, No. 1G, October 1956, isdevoted in major part to a more recent survey of the uses andcharacteristics of ferrites.

Included among the new transmission components that have foundwidespread use in the microwave art is the so-called isolaton Theisolator may be defined as a circuit element which is transparent toelectromagnetic waves propagating therethrough in one direction,designated the forward direction, whereas electromagnetic wavespropagating in the opposite, or reverse, direction are attenuated by theisolator to the extent required by the system.

In the above-mentioned article by Fox, Miller and Weiss three distinctclasses of isolators are described. l'hese include the Faraday rotationisolators, the fielddisplacernent isolators and the resonance isolators,The advantages and limitations of the several types disclosed aredescribed in an article by C. Bowness entitled Microwave Ferrites andTheir Application, published in the July-August 1958 issue of theMicrowave Journal, Volume l, page 13. ln brief, all three classes arefound to be temperature and frequency sensitive and, in addition, thethin resistive varies or lossy strips used in conjunction with theFaraday rotation isolators and the field displacement isolators, moreparticularly the latter, tend to unduly limit their power handlingcapabilities.

It is therefore the object of this invention to introduce nonreciprocalattenuation over a broader band of temperature and frequency variationsand at higher power levels.

Recognizing that in the presence of gyromagnetic materials the waveguidemodes differ radically in almost every essential detail from thepropagating modes normally `associated with the conventional unloadedwaveguide, it is proposed to utilize these anomalous modes to producenew and useful results. In particular, it is proposed to utilize thatcertain class of higher order modes for which the energy tends toconcentrate about an interface or boundary of the gyrornagnetic medium.This concentration about the boundary produces extremely large radiofrequency magnetic field densities in a relatively small portion of thegyromagnetic materials. Within this region of the material there isinduced, by this class of modes, highly turbulent electron spin systemshaving extreme variations in the alignment of the magnetization vectorsassociated with such spin systems. In this nonuniformly induced state ofalignment of the magnetic spins there is a greater tendency for thematerial to absorb radio frequency energy, resulting in what may bereferred to as a self-loss, nonresonant attenuator. lt

is self-loss in the sense that power absorption takes place in thegyromagnetic medium itself rather than in some external lossy material,and it is nonresonantl in that it operates at -a direct current magneticbiasing field intensity far below that required to induce the usualresonance conditions. This mode of operation is in marked contrast tothe arrangement of the spin systems as they exist in the usual resonanceattenuators wherein substantially all the magnetization vectors arealigned parallel to the direct current biasing field.

While it is recognized that imperfections in any practical transmissionsystem will tend to induce higher order modes of the type hereinconsidered, the prior art has arduously sought to minimize this tendencyby appropriately shaping and proportioning the gyromagnetic materials.Attenuation of microwave energy has been achieved by either usingexternal lossy materials in association with the gyromagnetic materialor by resonantly biasing the gyromagnetic material itself. By contrast,it is the purpose of this invention to produce maximum disruption of thenormal propagating modes by inserting discontinuities in the wave pathin the region of the gyromagnetic material and thereby to convertsubstantially all of the wave energy to higher order mode energy. Thesehigher order modes being bound very tightly as surface waves to theinterface `of the gyromagnetic material are then highly attenuated dueto the very inefficient use of said material as a transmission medium. Acrude analogy of the operation of an attenuator in accordance with thepresent invention would be to compare the loss induced in this type ofattenuator to that induced in the conventional conduction system whenlarge currents are caused to flow through conductors having extremelysmall cross-sectional dimensions.

It is ytherefore a more specific object of this invention to introduceattenuation in electromagnetic wave sys tems by inducing a high degreeof nonuniformity in the electron spin systems of gyromagnetic materials.

It is a further object of this invention to induce such nonuniformity byconcentrating, into a restricted region, the radio frequency magneticfields of the higher order turbulent modes associated with suchmaterials.

lt is another object of this invention that such attenuation benonreciprocal.

In accordance with the broad principles of the invention, the intensityand distribution of the higher order turbulent modes characteristic ofthe transmission modes in gyromagnetic media are greatly enhanced bymeans of scattering elements longitudinally distributed along thegyromagnetic medium.v The transmission path and the igyromagneticmaterial are so shaped and oriented with respect to each other as tominimize the effect of the scattering elements for one direction ofpropagation and to enhance it for propagation in the reverse direction.

ln a preferred embodiment of the invention, the two elements of atwo-element transmission path are separated over a longitudinallyextending region by a magnetically polarized gyromagnetic medium. One ofthe elements is smaller than the other of said elements, producing arelatively high field density in the region of the smaller element andadjacent to one of the interfaces of the gyromagnetic material.Scattering means are longitudinally distributed along the interfaceadjacent to the smaller element. For one direction of propagation alongthe path,

the higher order modes introduced by the scattering means areconcentrated about the adjacent interface of the gyromagnetic materialand are attenuated. For propagation in the opposite direction, thehigher order modes tend to concentrate about the far interface of thegyromagnetic material, but because of the distance from the scatteringelements the coupling is small and the resulting attenuationcorrespondingly small.

An isolater constructed in accordance with the invention differsfundamentally from the prior art isolators in that its nonreciprocaloperation does not depend upon some secondary characteristic of thetransmission path created through the direction of propagation of thewave energy, but rather it is an inherent characteristic of the actionof the turbulent mode on the gyromagnetic material itself, and dependsAdirectly upon the direction of propagation of the wave energy. rThus,by properly shaping and proportioning the two elements and thegyromagnetic material, the attenuation can be controlled as a functionof the direction of wave propagation to obtain an isolator having a highreverse-to-forward loss ratio.

It is a feature of the invention that it operates over a broad frequencyband and is substantially insensitive to changes in the operatingtemperature or frequency. It is a further feature of the device that itmay be scaled down to very small physical dimensions in a givenfrequency range, there being no cut-ofi characteristic associated withthe device, i.e., there is no requirement that the transverse dimensionsof the structure be comparable to a wavelength of the frequencies ofinterest.

These and other objects and advantages, the nature of the presentinvention, and its various features, will appear more fully uponconsideration of the various illustrative embodiments now to bedescribed in detail in connection with the accompanying drawings, inwhich:

FIG. l is a perspective View of the invention showing the scatteringmeans in relationship to the gyromagnetic material;

FIG. 2 shows, by way of illustration, the iield distribution of adielectric loaded waveguide in a cut-off state;

FIG. 3 shows, by way of illustration, the iield distribution in ananisotropic wave transmission path;

FIG. 4 shows, by way of illustration, the electric eld distribution inthe gyromagnetic material;

FIG. 5 shows one possible -type of uniformly distributed scatteringmeans; and

FIG. 6 is a perspective view of the second embodiment of the inventionshowing alternate scattering means, shaping of the gyromagneticmaterial, and lthe use of heat sinks embedded in the gyromagneticmaterial.

In FIG. l there is shown an attenuator inaccordance with the inventioncomprising a conductive channel, whose cross-sectional dimensions can besmall compared to a wavelength, as will be explained in greater detailhereinafter. Suitably supported within channel 10, by beads or othermeans not shown, is a thin conductive member il. Member 1i extendslongitudinally within channel 10 and together with the wide and narrowwalls thereof constitute a two-wire Wave supporting structure lil-#11.For convenience, hereinafter, all the two-wire systems described will bereferred to as strip transmission lines or simply lines. Y

Located to one side of member 11 and extending the full height ofchannel and the full width between member l1 and channel 16 is anelement 12 of material capable of exhibiting gyromagnetic propertiesover a range of operating frequencies of interest. The term gyromagneticmaterial is employed here in its accepted sense as designating the classof magnetically polarizable materials having unpaired spin systemsinvolving portions of the atoms thereof that are capable of beingaligned by an external magnetic polarizing iield and which exhibit asignificant precessional motion at a frequency within the rangecontemplated by the invention under the combined influence of saidpolarizing field and an orthogonally directed varying magnetic iieldcomponent. This precessional motion is characterized as having anangular momentum, a gyroscopic moment and a magnetic moment. Typical ofsuch materials are ionized gases, paramagnetic materials andferromagnetic materials, the latter including the spinels such asmagnesium aluminum ferrite and aluminum zinc ferrite, and thegarnet-like materials such as yttrium iron garnet.

Element l2, is biased by a steady magnetic eld at right angles to thedirection of propagation of wave energy along the line. As illustratedin FIG. l, 'this ield may be supplied by a single solenoid structurecomprising a magnetic core i3 having pole pieces N and S bearing againstthe top and bottom wide walls, respectively, of channel 1). Turns ofwire 14 on core i3 are connected through switch and rheostat i6 to asource of magnetizing current 17. The biasing eld, however, may besupplied by an electric solenoid with a magnetic core of other suitablephysical design, by a solenoid without a core, by a permanent mag netstructure, or element ll may be permanently magnetized if desired.

The amplitude of the biasing field is a function of the signalbandwidth. In particular, .the relationship between the biasing ield andthe signal frequency is given by where wh is the upper limit of thefrequency band; w1 is the lower limit of the frequency band; 'y is equalto 2.8 106 megacycles per oersted; M is the saturation magnetization,and

H is the effective internal biasing field.

The essence of the present invention resides in the discontinuities 1Sdistributed along the surface of member 1i nearest the gyromagneticmaterial l2.

In applicants paper entitled Character of Waveguide Modes inGyromagnetic Media published in the Bell System Technical Journal,Volume 36, March 1957, on pages 409-426, the transmission of wave energyin isotropic and in anisotropic waveguides is considered. While thisarticle deals specifically with waveguide structures, the nature of theenergy distribution of the higher order modes Within the gyromagneticmaterial is equally applicable to the two-conductor transmission systemof FIG. 1. In this article it is shown that in the presence ofgyromagnetic materials the waveguide modes diler radically in almostevery essential detail from the propagating modes normally associatedwith the conventional unloaded or isotropic waveguide.

When it is stated that the higher order modes of a guide of restrictedsize are in a cut-oit state, it means that there is a longitudinallydecaying field associated with the cut-olf modes, but that thetransverse crosssection of the guide has associated with it waves of anharmonic nature, at least over restricted regions. Such a fielddistribution is shown in FIG. 2 in which there is shown a waveguide 20partially filled with dielectric material 21. Here, at least over thedielectric filled section, there are observed transverse harmonic energydistributions. These are represented by vertical variations 22 andhorizontal variations 23. The longitudinally decaying iield normallyassociated with the cut-off state is shown by means of curve Z4.

'In an anisotropic wave transmission path, the wave energy distributionis inverted. That is, cut-olf may be induced with respect to one or bothof the transverse dimensions of the path, while retaining the harmoniccharacter of the wave in the longitudinal distribution. Incontradistinction to the conventional guide, the energy associated withthese higher order modes propagates within the path. This situation isillustrated in FIG. 3 in which there is shown a waveguide 30, partiallyfilled with some gyromagnetic material 31. The vertical harmonicvariation is given by curve 33, and the longitudinal harmonic variation,representing a propagating wave, is given by curve 32. The cut-offhorizontal transverse distribution is represented by the exponentiallydecaying wave 34.

There remains, however, one degree of comparison or similarity betweenthe modes of the two differently loaded guides. This similarity residesin the fact that cut-off, whenever induced by the higher order modedistribution, becomes progressively sharper the greater the order of themodes present. This relates equally to the horizontal transversedistribution 34 of FIG. 3 irrespective of the fact that it is atransverse rather than a longitudinal distribution. rl`hus, withincreasing order of distribution, the energy is more tightly boundptothe interface of the gyromagnetic material. Since this energy ispropagating, the transmission of these higher order modes becomesincreasingly inefficient as a consequence of the highly restrictedportion of the guide through which the energy is being channeled.

The problem of stimulating the higher order modes is greatly simplifiedif the gyromagnetic material is initially located in a region of thewave path having a high density, nonuniform eld distribution. Theseconditions are fairly well met in a two-element transmission path of thetype shown in FIG. 1. As may be seen by referring to FIG. 4, theelectric field lines il in such a structure are densely concentrated atthe boundary of the inner conductor l1, diverge within the gyromagneticmaterial 12 and terminate on the walls 10. Thus, within the gyromagneticmaterial 12, the electric field lines have those preferredcharacteristics which tend to facilitate the generation of the higherorder modes of the class heretofore described.

If the energy interchange process between the line and the gyromagneticmaterial is thought of in terms of a perturbation phenomenon, the devicemay be considered as consisting of a pair of coupled transmission paths.The first path, comprising the strip transmission line, is a low losspath having a velocity of propagation substantially that of free space,whereas the second path, comprising the incoherently aligned electronspin systems of the gyromagnetic material, is a high loss path having avelocity of propagation an order of magnitude or more slower than thestrip transmission line.

Because of the large difference in phase velocities there is, to a firstorder, no interaction between the two transmission systems. 'lhissituation can be effectively altered by interrupting the longitudinalsymmetry of the device in the regions of high current densities. For thetwoelement embodiment shown in FIG. 1, high current densities occur overthe region in which conductor l1 and material 12 are nearest each other.As shown, the symmetry is interrupted by placing irregularities alongthe surface of element l1 immediately adjacent to the gyromagneticmaterial. 'Ihese irregularities or interruptions of random size anddistribution create the higher order space harmonics, which, asindicated above, tend to concentrate their wave energy at a boundary ofthe gyromagnetic material. in the presence of these higher order spaceharmonics a sympathetic interaction is established between the twotransmission system. Energy is scattered at the interface into higherand more complex modes, a process which facilitates the repeatedextraction of energy from the two line system at one phase velocity, andtransfers it to the gyromagnetic transmission system at another phasevelocity. The energy transferred from the line to the gyromagneticmaterial now finds itself in a far lossier medium and this energy isquickly dissipated.

In a broad-band system, the irregularities are randomly distributedalong conductor 1l as in FIG. 1 and have cross-sectional areas which arealso randomly varied both as to size and shape. The random nature ofboth the distribution and the configuration of the scattering elementsis intended to avoid specific space harmonic selection which mightcreate excessive frequency sensitivity. Hence, the discontinuities orinterruptions of whatever nature are made aperiodic. On the other hand,in a narrowband system, the attenuation per unit length of ferrite maybe increased by careful selection of the higher order modes generated.In the narrow-band situation, the scattering elements, as shown in FIG.5, are uniformly distributed to provide periodic interruptions oftheincident Wave. Thus, in FIG. 5, indentations of equal size and shape arelongitudinally distributed along element lll. The indentations arephysically separated from each other by the longitudinal intervals 5l.The periodic spacing tends to favor the space harmonics associated witha relatively narrow band of frequencies, thus making the attenuator morefrequency sensitive than the random arrangement of FIG. 1.

The higher order modes have been characterized as surface waves bound tosome interface of the gyrornagnetic material. It may be shown fromanalysis that, over a given frequencyrange and in the presence of agiven polarizing field Hdc, the energy in the gyromagnetic material isprimarily bound to the far surface 19, adjacent to the wall of channellil, for wave propagation in the forward direction, while it isprimarily bound to the near interface 20, adjacent conductor l1, forpropagation in the reverse direction. The energy coupled between thestrip transmission line mode and the modes in the gyromagnetic materialat either interface is related to the integrated product of the relativefields associated with each of these modes. Since the overlap of themode fields is very large in the reverse direction because of theproximity of interface 263` to the conductor 11, a significant couplingof energy occurs, which energy is eventually dissipated in thegyromagnetic material. Conversely, the overlap of the fields is verysmall for propagation in the forward direction because of the relativelylarge distance between surface 19 and the conductor l1. Consequently,the energy exchange and the resulting attenuation are far less forpropagation in the forward direction than for propagation in the reversedirection. As a consequence, the operation of an attenuator soconstructed is nonreciprocal.

FIG. 6 shows an alternate embodiment of the invention in which the modeconversion mechanism has been altered and in which the isolation ratiois improved and other minor changes made. Thus, for example, because theattenuation is not particularly affected by the width s of conductor 1lin FIG. l, it has been replaced in the embodiment of FIG. 6 by a smoothcylindrical wire 6l. In the embodiment of FIG. l, the non-uniformity inthe system which produces mode conversion resided in changes in theboundary conditions along conductor 1-1 in the region of thegyromagnetic material. In the embodiment of FIG. 6, mode conversion isinduced by causing changes in the dielectric 'constant of the wave pathin the region of the gyrornagnetic material. This is accomplished byreplacing the single element of gyromagnetic material with numeroussmaller segments 62 of a gyromagnetic material having a high dielectricconstant such as, for example, one of the ferrites, which arelongitudinally distributed along the wave path. The effect of breakingup the gyromagnetic material in the manner shown is to allow thesurrounding dielectric material (in this embodiment, air) to fill theregions 63 between segments 62;. Because of the large difference in thedielectric constants between ferrite and air, substantial local changesin the effective dielectric constant of the wave path are established inthe immediate vicinity of the ferrite material. The overall effect is tocreate, by means of such dicontinuities in the wave path, a plurality ofdielectric scattering elements along the inner surface of elements 621.Obviously, the choice of dielectric material used must be considered inrelationship to the type of gyromagnetic material used in order toproduce the desired local changes in the dielectric constant of the Wavepath over the region of interest.

The purpose of tapering the segments 62 is to reduce the forward loss ofthe isolator by making it more difficult for the higher order modes totend tol be established on the far interface. While the forward lossesin this type of isolator tend to be small, they are nevertheless nite.lt would be desirable to be able to reduce them still further ifpossible. This can be done by increasing the difference in the phasevelocity between the low loss strip transmission line and the high losspath which, for the forward direction is that portion of the ferritenear the far interface. Since the phase velocities of the high ordermodes vary directly as the height of the gyromagnetic material in thevicinity of the interface and inversely as the mode order, decreasingthe height for a given mode, decreases the already slower mode phasevelocity in the gyromagnetic material still further. Thus, as thedisparity in the relative phase velocities in the two paths increases,the attenuation per unit length decreases. The near interface, however,being the full height of the channel, is unaffected, and coupling tothis surface, in the reverse direction, is not reduced.

Because the wave energy concentrated within the gyromagnetic material isso intimately associated with its boundary, conductive heat sinks may beimbedded within the material to improve the heat dissipating propertiesof the -attenuator and increase its power handling capabilities. Such anarrangement is also included in FG. 6 wherein a series of conductiveposts 64 extend from conductive Iwall 65 of channel 60 to about halt'way into each of the segments 62. Energy attenuated within the materialgenerates heat which is conducted by the posts 64 to conductive surface65 and dissipated to the surrounding environment. This has the eect ofreducing the size of the isolator for a given power level or increasingthe permissible power level for a given size isolator. It is obviousthat such an arrangement can be used with other embodiments of this typeof isolator.

It had been mentioned earlier that an isolator of the type describedherein could be made extremely small compared to a wavelength. Thisresults from the fact that the strip transmission line, being atwo-conductor system, does not have a cut-cti frequency in the sensethat a waveguide transmission path does. Hence, the dimensions of thestrip transmission line (or low-loss path in this type isolator) can bemade extremely small. Similarly, the higher order modes in thegyromagnetic material (which comprises the high-loss path) are alsosubstantially independent of the free-space wavelength. It is rather thedistribution of these modes in the gyromagnetic material that providesthe nonreciproeal losses, and these tend to maintain a constant ratiothat is substantially independent of guide size. However, there is alimitation which was mentioned with regard to the structure of yFlG. 6and which must be considered in designing small attenuators. Asmentioned earlier, reducing the height of an interface increases therelative diierence in' phase velocity between the wave energypropagating along the strip transmission line and the higher order modeswhich tend to establish themselves at the surfaces of the gyromagneticmaterial. The phase velocity of the wave energy propagating along thestrip transmission line remains substantially constant as its sizedecreases. However, the phase velocity of the higher order modesdecreases with decreasing height. This additional difference in phasevelocities tends to reduce the coupling from the strip transmission linemode to the higher order modes. Thus, while the reversetoforward lossratio remains substantially constant, the attenuation per unit lengthdecreases as the size of the isolator decreases. The design of theisolator is, therefore, a matter of compromise between cross-sectionalsize and length, with any particular design depending upon theparticular application at hand.

in all cases it is understood that the above-described arrangements areillustrative of a small number of thc many possible specihc embodimentswhich can represent applications ol the principles of the invention.Numerous and varied other arrangements can readily #be devised inaccordance with these principle-s by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

l. An isolator for electromagnetic wave energy com* prising atwo-element transmission path supportive of said wave energy in the TEMmode of wave propagation over a range of operating frequencies, one ofsaid elements having a surface area substantially smaller than thesurface area of; the other of said elements, an element of magneticallypolarizable material exhibiting gyromagnetic eltects over said frequencyrange and supportive of a class of higher order modes of wavepropagation different than said TEM mode disposed between'at least aportion of said unequal surface areas, means for mag-l netically biasingsaid material in a direction transverse to the direction of wavepropagation along said path and means for coupling substantially all ofsaid wave energy `from said TEM mode of wave propagation to said higherorder modes comprising a plurality o sub stantially lossless electricaldiscontinuities longitudinally distributed along the interface of saidmaterial adjacent to said smaller surface.

2. The combination according t0 claim 1, wherein said discontinuitiescomprise a plurality of successive changes in the boundary conditionsalong said smaller surface.

3. In an electrical system a two-conductor transmission path supportiveof electromagnetic wave energy comprising a conductively boundedenclosure having a longitudinally extending conductive member disposedtherein, an element of magnetically polarizable material exhibitinggyromagnetic eiiects over the operating frequency range of said pathpartially lilling said enclosure asymmctrically located within said pathbetween said member and a portion of the inner surface of saidenclosure, means for magnetizing said element in a direction transverseto the direction of wave propagation and means for inducing a system ofhigher order modes concentrated along one of the boundary surfaces ofsaid element comprising a plurality of irregularities longitudinallydistributed along said conductive member over an interval coextensivewith said element, said inducing means and said material being incoupling relationship to dissipate within said material substantiallyall of said wave energy.

4. An isolator comprising a section of two conductor transmission linesupportive of a given mode of wave propagation having a solelytransverse electric and a solely transverse lmagnetic. eld distribution,the first of said conductors having a sur-face area substantiallygreater than the surface'area of the other of said conductors, means forapplying wave energy to said line in said given mode having a firstvelocity of propagation, and a plurality of magnetically polarizedelements of gyromagnetic material longitudinally distributed along aregion of said line disposed between a portion `of the sur-faces of saidtwo onductors, said elements being supportive of a class of higher ordermodes of wave propagation having propagation velocities substantiallydifferent than said iirst velocity of propagation, said plurality ofelements comprising means for coupling substantially all of said waveenergy from said given mode of wave propagation to said higher ordermodes of wave propagation to dissipate within said elements the energyassociated with said higher order modes for only one direction ofpropagation.

5. The combination according to claim 4 wherein each of said elements ofgyromagnetic material has a transverse dimension in that portion of saidregion adjacent to said first conductor that is substantially smallerthan the transverse dimension of each of said elements in that portionof the region adjacent to said other conductor.

6. The combination according to claim 4 wherein said elements havesubstantially the same size and shape and are uniformly distributedalong said path.

7. The combination according to claim 4 wherein said elements are ofunequal size and shape and are aperiodically distributed along saidpath.

8. The combination according to claim 4 wherein at 9 least oneconductive post extends into each of said ele- 2,849,683 ments, saidposts connecting to the rst of said conductors. 2,849,684 2,922,125References Cited in the le of this patent UNITED STATES PATENTS 52,777,906 shockley Jan. 15, 1957 13,223 2,834,947 Weisbaum May13,1958 61 10 Miller Aug. 26, 1958 Miller Aug. 26, 1958 Suhl Jan. 19, 1960FOREIGN PATENTS Australia Aug. 6, 1958 France Sept. 15, 1958

